1. Field of the Invention
This invention relates to a tapered rod producing process for working a stock in the form of a straight wire or bar into a tapered rod which is used as a taper coil spring for an automobile, a railway vehicle or the like.
2. Description of the Prior Art
Various processes and types of equipment for producing a tapered rod of the type mentioned are conventionally known. One of such conventional types of equipment is disclosed, for example, in Japanese Patent Laid-Open No. 60-56416 and Japanese Patent Laid-Open No. 60-56417, published Apr. 2, 1985. The equipment is illustratively shown in FIG. 1.
Referring to FIG. 1, the equipment shown is characterized in that it comprises two or more pairs of constant speed feed rollers 2 for holding and feeding a metal wire stock 1 at a constant speed, a heating device 3 located downstream of the constant speed feed rollers 2 in a direction of the metal wire stock 1 being fed, a cooling device 4 located further downstream of the heating device 3, two or more pairs of variable speed tension rollers 5 located further downstream of the cooling device 4 for holding and feeding the metal wire or bar stock 1 at a speed higher than the feeding speed of the constant speed feed rollers 2, and a speed controlling device 6 for controlling the speed of the variable speed tension rollers 5.
In this assembly, the feeding speed Vi of the variable speed tension rollers 5 is gradually accelerated or decelerated while remaining higher than the feeding speed Vo of the constant speed feed rollers 2 in order to provide a portion of the metal wire stock 1 between the heating device 3 and the cooling device 4 with tensile plastic deformation to work the straight metal wire stock 1 into a tapered rod 7.
The feeding speed Vi of the variable speed tension rollers 5 for providing a straight metal wire stock 1 with tensile deformation to work the metal wire stock 1 into a tapered shape is controlled under a hypothesis as illustrated in FIG. 2.
In particular, where the feeding speed of a metal wire stock 1 being supplied to the heating device 3 is denoted by Vo which is equal to the speed of the constant speed feed rollers 2, the sectional area of the metal wire stock 1 is denoted by Ao, the sectional area of the tapered rod 7 after the metal wire stock 1 has been heated, plastically worked and then cooled by the cooling device 4 until it has no more plastic deformation is denoted by Ai, and the feeding speed of the tapered rod 7 with the sectional area Ai is denoted by Vi, the following relation stands; EQU Ao.times.Vo=Ai.times.Vi
Accordingly, if the speed Vi of the variable speed tension rollers 5 is controlled in accordance with a relation ##EQU1## then a tapered rod 7 in which Ai gradually increases or decreases can be produced.
When variable speed control of the variable speed tension rollers 5 is considered under such a premise, it can be understood that the variable speed control should be accomplished by a combination of a straight line and simple curves in a V-t chart as shown in FIG. 3. It is to be noted that, in FIG. 3, V represents a feeding speed of a tapered rod 7 by the variable speed tension rollers 5, and t denotes time.
However, if a metal wire stock is actually worked to produce a tapered rod under such a variable speed control as illustrated in FIG. 3, there is a problem that a product of an a desired shape cannot be obtained in that, relative to an a desired shape indicated by broken lines in FIG. 4, the tapered rod becomes thicker within an L.sub.1 tapered section (a section in which the diameter of the metal wire stock gradually decreases from the downstream side toward the upstream side in the feeding direction of the metal wire stock, the section being hereinafter referred to as "wire diameter gradually decreasing portion") so that the taper will be shorter with a substantially large gradient while the tapered rod becomes thinner within another tapered section L.sub.2 (a section in which the diameter of the metal wire stock gradually increases from the downstream side toward the upstream side in the feeding direction, the section being hereinafter referred to as "wire diameter gradually increasing portion") so that the taper will be longer with a substantially small gradient as indicated by solid lines in FIG. 4.
The inventors of the present patent application have made various investigations through several experiments and found out that, in actual tapered working, plastic deformation and its fixation of a metal wire stock do not occur suddenly as in the hypothesis of the model of FIG. 2 but deformation starts at a point further upstream as illustrated in FIG. 5 and that a so-called deforming zone 8 between the deformation starting point and a deformation ending point has an influence upon a profile of a taper of a product.
In particular, if a metal wire stock being worked is instantaneously stopped to observe a profile of a heated portion of the stock in detail in order to examine a deforming condition of the stock in the deforming zone 8, the profile observed is such as illustrated in FIG. 6 and is thus quite different from that of the model of FIG. 2. As seen in FIG. 6, deformation of the heated stock starts at a rear half portion of the heating device 3 and continues to the inside of the cooling device 4.
On the other hand, where a metal wire stock is, for example, made of steel, it is normally heated to a temperature higher than 750.degree. C. for working. Where such heating is advantageously used to heat treat the steel wire stock for hardening, it is heated to a temperature higher than 900.degree. C.
However, it is known that, if a wire stock is worked at such a high temperature, fine cracks having depths of several microns or so will appear along crystal grain boundaries of a surface layer of a product worked from the wire stock.
Then, if a product having such cracks is worked into a part such as, for example, a coil spring which will undergo repetitive loads, when the part is used, it is forecast that the cracks may accelerate fatigue of the product and consequently deteriorate the strength of the product. Accordingly, such a product is not preferable.